This invention relates to packing elements for heat exchange and mass transfer between liquid and gas phases of fluid, for use in, for example, cooling towers. In particular this invention relates to regular or ordered packing elements formed from formable sheet material and having corrugations.
A common object in the design of any cooling tower pack is to provide a pack with efficient heat exchange and mass transfer characteristics. In fulfilling this object it is important to maximise the surface area to volume ratio of liquid on the pack and hence to effect an even distribution of fluid over the pack. Uniform wetting is easily achieved in self-wetting woven wire fabric elements where capillary forces act between the fibres. One drawback of such elements is that they are expensive to produce. In contrast, elements made from formable sheet material such as Polyvinylchloride (PVC), Polythene etc. are much cheaper to produce. However, designing a packing element from sheet material which achieves a uniform distribution of liquid over the surface of the sheet and thus efficient mass transfer from bulk liquid to film, is not a trivial matter, especially since capillary forces do not act between the liquid and sheet element as in the fabric elements.
Forming the sheets with folds or corrugations represents the most common method of increasing the surface area. Liquid flowing over the sheets is spread laterally either by inclining the corrugations at an angle to the vertical or by forming the sheets with secondary perturbations, or by incorporating in the sheet a combination of both.
Regular packing elements consisting of corrugated sheets fixed together to form an array of parallel vertical passageways, such as in GB-A-2,093,967 and EP-A-28545, have a number of advantages over other known packing arrangements.
In a conventional cooling tower, liquid is sprayed onto the pack from an array of horizontal plan sprays located above the pack. Packs whose passageways are disposed at an angle to the vertical have shadowed areas at the entrance of the pack which cannot be wetted effectively, and in the case of vertically falling liquid cannot be wetted at all. Thus, an advantage of packs having vertically standing passageways is that every unit area of the entry faces has an equal probability of being wetted.
In packs with slanted corrugations the liquid tends to concentrate into channels defined by the corrugations, resulting in a non uniform distribution of liquid over the pack. Furthermore, liquid flowing over the corrugations is for part of the time suspended from above, increasing the probability of the liquid becoming dissociated from the surface of the sheet. This loss of liquid results in a loss of thermal performance. In contrast, packing elements having corrugations vertically aligned do not suffer from either of these intrinsic problems.
In natural-draft cooling towers operating in counter-flow mode the air flow is governed by the relation between the buoyancy forces and the pack impedance. A pack with vertical passageways will present less impedance to the air flow than packs which are inclined to the vertical, because the passageways run parallel to the air flow direction. By the same argument, packs with horizontally disposed passageways present less impedance to the air flow in cross-flow cooling towers.
A still lower impedance is achieved for packs in which both the cross-sectional geometry of the corrugations forming the passageways and their cross-sectional area remain constant over the length of the pack. The impedance is further reduced if the passageways formed by the corrugations are linear.
A lower impedance results in a lower pressure drop along the length of the packing element for a given air flow and, therefore, in a natural draught cooling tower, a larger air flow for a given buoyancy induced driving force. This results in a more efficient heat exchange and mass transfer between the liquid and gas phases across the boundary layer for several reasons. Air flowing next to the boundary layer at any given point will be cooler and less saturated than air having a lower velocity, giving a larger thermal gradient across the layer resulting in a correspondingly higher rate of heat exchange. The faster air flow also promotes the diffusion of saturated air away from the boundary layer next to the water film, thus enhancing the evaporation rate, cooling the liquid further.
A further advantage of vertically disposed corrugated sheets is that they have considerable strength in the vertical, i.e. load bearing direction.
Although packs comprising vertical corrugated sheets have certain advantages as mentioned above, there remain a number of practical problems to be overcome, which may be solved with an appropriate design.
As mentioned above, it is common to employ an array of horizontal plan jets to spray the liquid onto the pack. A common problem associated with this water distribution apparatus is that liquid impinging on the top of the pack will have an uneven coverage across the pack. An object of the present invention is to provide a packing element which promotes fast and effective distribution of liquid from the top of the pack over a relatively short vertical distance, and thereafter to maintain an even coverage as the liquid continues to propagate.
The liquid coolant in many cooling tower applications is water drawn from rivers, and is preferred because it is readily available and therefore cheap. This water is usually contaminated with suspended particles of sedimentary deposits as well as other particulate or viscous contaminants. Use of such water in unpurified form exposes the packing elements to the possibility of fouling. Initially the deposits will be trapped at points in the surface structure where the liquid is caused to stagnate. In such regions the particles will tend to be deposited and build up on the surface thus enhancing the stagnation region and increasing the deposition. In time, this will cause essentially two problems. The deposits will form a layer, tending to smooth over any surface structure, so that liquid is deflected to an ever decreasing degree with the result that the patterned surface loses its effectiveness in distributing the liquid. In the worst case the layer would destroy all surface structure, leaving a planar sheet. This would drastically reduce the efficiency of the pack as well as increase its overall weight. Furthermore, the build-up of fouling will begin to restrict the free space between neighbouring sheets, thereby increasing the impedance to the air flow. This causes a reduction in air flow with consequent loss of performance of the pack. Finally, if the fouling is great enough, the weight of it will compromise the structure of the cooling tower.
Another problem in designing a cooling tower pack is to provide an appropriate surface structure which compels the falling liquid to reside on the sheet for an adequate period of time (dwell time), so that the liquid is cooled effectively and sufficiently from the moment the liquid enters the pack to the time it reaches the bottom of the pack. Appropriate surface structure is essential in achieving this, but surface structure tends to promote fouling. Thus, there is a trade-off between optimising the dwell time and reducing the possibility of fouling to a minimal extent.
It is a further object of the present invention to provide a pack in which the dwell time of liquid on the pack can be optimised whilst at the same time the possibility of fouling by deposits can be minimised, thereby prolonging the effectiveness and lifetime of the pack.
The problem to be solved is to provide a cooling tower pack which fulfils all of the above mentioned objects.